[CVE-2026-53212][MODERATE 7.0] netfilter: nft_tunnel: fix use-after-free on object destroy

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-53212
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nft_tunnel: fix use-after-free on object destroy
Commit: 349df61526d2e39decc685d246202e3e284cfe05
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=349df61526d2e39decc685d246202e3e284cfe05
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53212
Analysis date: Thu, 25 Jun 2026 07:04:21 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A UAF in nft_tunnel object destruction can leave queued packets with dangling dst references after metadata_dst is freed, allowing a local actor with nftables control to trigger a kernel crash and potentially limited memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-53212 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53212

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53212	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in nft_tunnel object destruction because metadata_dst_free() frees the metadata_dst object directly while packets may still hold dst references obtained through nft_tunnel_obj_eval. Packets queued in a qdisc such as netem can keep a reference after the nft tunnel object is destroyed, leaving a dangling pointer that is later used by dst_release during packet dequeue. For the CVSS the PR:L is used because practical triggering requires local ability to configure or destroy nftables objects, commonly CAP_NET_ADMIN or an equivalent delegated network administration capability. The issue is not directly network reachable as an unauthenticated packet only attack, although packet queuing is part of the trigger sequence. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because this is a real use after free in a refcounted kernel networking object.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER INIT NETFILTER UAF QDISC KPANIC PACKET 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The patch fixes a real UAF where `metadata_dst_free()` frees an object while packets may still hold `dst_entry` references.
* Real lifetime corruption: +1. This is a refcount/lifetime mismatch between `metadata_dst_free()` and `dst_hold()`/`dst_release()`.
* Reliable kernel crash / strong DoS: +1. Queued packets later call `dst_release()` on freed memory, plausibly causing crash or memory corruption.
* Common networking / netfilter path: +1. The bug is in nf_tables tunnel object handling and interacts with packet queuing.
* Requires CAP_NET_ADMIN in typical deployments: -2. Creating/destroying nftables tunnel objects and configuring qdisc/netem generally requires network administration privileges.
* Hard timing or special sequencing: -1. Triggering requires packets to retain dst references while the nft tunnel object is destroyed.

Paranoid additional interpretation:

* Local delegated trigger: +1. In container or namespace setups, CAP_NET_ADMIN may be delegated to a reduced privilege actor rather than full host root.
* Weak LPE concern: +1. The UAF reaches `dst_release()` on freed memory in a refcounted networking object, so limited corruption beyond pure DoS is plausible even without a demonstrated reclaim primitive.

## 3. Reachability analysis

The issue is not remotely reachable as a pure unauthenticated packet parsing bug. Practical triggering requires local control over nftables tunnel objects and packet queuing behavior, usually CAP_NET_ADMIN. Namespaces and containers matter because CAP_NET_ADMIN may be delegated to container root or service accounts, making PR:L a reasonable paranoid interpretation rather than full PR:H. The path is in netfilter/nf_tables, which is widely deployed, but the specific tunnel object plus queued packet lifetime condition is more specialized.

## 4. Severity interpretation

This is not an ordinary resource leak or warning-only bug. It is a concrete UAF caused by freeing `metadata_dst` without respecting outstanding `dst_entry` references. Realistic impact is at least local DoS via kernel crash. Practical privilege escalation is not demonstrated, because the patch does not show attacker-controlled reclaim, controlled overwrite, callback dispatch, or type confusion. However, because the freed object is later touched through `dst_release()`, paranoid triage should keep it as Actionable Moderate and require manual review.

## 5. One-sentence report phrase

A UAF in nft_tunnel object destruction can leave queued packets with dangling dst references after metadata_dst is freed, allowing a local actor with nftables control to trigger a kernel crash and potentially limited memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF in a common networking subsystem with namespace-sensitive privileges and plausible impact beyond simple DoS, even though a practical LPE primitive is not demonstrated.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: netfilter: nft_tunnel: fix use-after-free on object destroy
Commit: 349df61526d2e39decc685d246202e3e284cfe05
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=349df61526d2e39decc685d246202e3e284cfe05

Commit description:

nft_tunnel_obj_destroy() calls metadata_dst_free() which directly
kfree()s the metadata_dst, ignoring the dst_entry refcount. Packets
that took a reference via dst_hold() in nft_tunnel_obj_eval() and
are still queued (e.g. in a netem qdisc) are left with a dangling
pointer. When these packets are eventually dequeued, dst_release()
operates on freed memory.

Replace metadata_dst_free() with dst_release() so the metadata_dst
is freed only after all references are dropped. The dst subsystem
already handles metadata_dst cleanup in dst_destroy() when
DST_METADATA is set.

Fixes: af308b9 ("netfilter: nf_tables: add tunnel support")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nft_tunnel.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=349df61526d2e39decc685d246202e3e284cfe05

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-53212 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53212

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 4
  Paranoid ActionableScore: 6

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).